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How to Determine Spectral Emissivities in Real Applications

The (spectral) emissivity ε is commonly defined as the ratio of the emitted radiation of a real object to the emitted radiation of a black body at the same temperature:

Spectral Emissivities in Real Applications





Every material will have a different emissivity, particularly when considering the surface and condition variants. For example, while a shiny metal surface typically has a high reflection, it will have a low emissivity. If this metal surface is coated or covered, it will have a much higher emissivity and a reduced reflection.

Figure 1 shows a thermal infrared image of electrical connections in a switch cabinet, taken in the spectral range of 8 to 14 µm. What is striking is that for the middle contact, two temperatures are measured, 36 °C and 84 °C. The actual wire insulation temperature is very similar to the actual metal connector crimp temperature, but the thermal image tells a different story.

Spectral Emissivities in Real Applications












Figure 1
(Click to enlarge)

The reason is that the very shiny metal connector crimp (which has a chrome surface) has a very low emissivity and high reflection. In contrast, the wire insulation has a very high emissivity and a low reflection, both at the same temperature.

In addition, the emissivity changes with the wavelength and the spectral response of the temperature measurement device. See Figure 2 below.

Spectral Emissivities in Real Applications










Figure 2
(Click to enlarge)

To obtain the correct object/surface temperature with a non-contact infrared temperature measurement device in real applications, the correct spectral emissivity needs to be known.

In some industrial processes where infrared temperature measurement is involved, the temperature reading is taken at an emissivity of 1 (100 %), resulting in a deviation between the absolute object temperature and the measured temperature. Suppose this measurement is taken under the same conditions and at the same emissivity. These measurements will be repeatable to each other and can be used for process control and logging. However, the challenge is determining the correct spectral emissivity in a real application to obtain the correct absolute object temperature.

To determine the emissivity, there are several common methods available. The most used method is to obtain a reference temperature, e.g., by taking a number of thermocouple measurements (contact measurement) and using this reference temperature (average of several readings) to correct for the emissivity of the infrared pyrometer, scanner, or thermal imager used.

Another possibility is to coat the object partly, take the temperature of the coated area (e.g. emissivity is around 95 %) and compare this reading with a measurement on the uncoated object surface in the immediate vicinity of the coated area (same temperature).

In some applications, the AMETEK Land Gold Cup pyrometer can be used (see Figures 3 & 4 below).

 Spectral Emissivities in Real ApplicationsSpectral Emissivities in Real Applications  
 Figure 3     
(Click to enlarge)                     
Figure 4
(Click to enlarge)
 


AMETEK Land founder Tom Land developed the first Gold Cup pyrometer, utilising the multiple reflections between the target object and a hemispherical mirror placed very close to the target object surface. By doing this, it is possible to collect nearly all radiation in the vertex of the mirror and then transmit that via fibre optic to a pyrometer detector to calculate the temperature.

This nearly results in black body conditions and absolute temperature measurement at 100% emissivity, enabling an accurate temperature measurement without any influence from emissivity change or background reflection. The Gold Cup pyrometer is often used to read reference temperatures and to determine real emissivities. This reading can be used as a reference temperature reading for pyrometers, scanners, or thermal imagers.

Typical applications, for example, in cold rolling and coiling of steel or aluminium strips with different emissivities, can be solved by measuring the wedge between the strip and the coil or the strip and a roll. Figures 5 & 6 show the AMETEK Land wedge measuring system and results and the increasing emissivity in a wedge by multiple internal reflections.

Spectral Emissivities in Real Applications











Figure 5
(Click to enlarge)

Spectral Emissivities in Real Applications










Figure 6
(Click to enlarge)

In 1984, researchers at Nippon Kokan, Japan, discovered that the cavity formed where a strip leaves a transition roll has a very high and stable emissivity, regardless of the emissivity value.

Because the cavity has an emissivity of almost 1 and ε = 1- γ, it means that the technique is also valid for heating sections where the background temperature is significantly higher than the strip temperature, such as vertical annealing lines.

The cavity formed between the strip and the roll is widely known as a wedge. The multiple internal reflections produced within it integrate to produce a black body environment. Further research conducted in 1986 by Land Infrared (today AMETEK Land) researchers found typical wedge emissivity was 0.995 (very near to 1). The wedge emissivity is also stable over a range of infrared wavelengths.

To sum -up, in real applications, typically, the spectral emissivity can be determined by:

  • Taking a number of reference temperatures, e.g., by using a thermocouple
  • Partly coating the object surface and comparing it with the uncoated surface in direct proximity
  • Determining the correct object temperature by using the AMETEK Land Gold Cup pyrometer
  • Measuring into a wedge at nearly 100 % emissivity (multi-reflection)

These are the most frequently used methods for determining spectral emissivities and measuring correct absolute temperatures in real applications, outside of laboratory emissivity determination, which is not practical in real applications.  The AMETEK Land experts are available to discuss your specific real applications and how to best determine spectral emissivities. 

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